The present invention discloses a single chip solution for a GMR (giant magneto-resistance) or MTJ (magnetic tunneling junction)-based sensor with a full 360-degree range capability for magnetic field measurement. This design greatly enhances sensitivity relative to prior art devices. It does not require an additional Hall sensor in order to provide a full 360 degree range of measurement. It may be implemented using either a GMR (giant magneto-resistance) or an MTJ (magnetic tunnel junction) device, which terms we will use inter-changeably in the description that follows.
As illustrated in FIG. 1, a GMR structure is deposited as a multi-layer structure starting with a seed-layer(11)/AFM(12)/AP2(13)/Ru(14)/AP1(15)/Cu(16)/free layer(17)/capping layer(18), where a ferromagnetic sub-layer AP2, non-magnetic spacer Ru and a ferromagnetic reference sub-layer AP1 form an anti-parallel synthetic pinned layer. In an MTJ structure, the Cu layer just below the free layer is replaced by an insulating tunnel barrier layer, (typically AlOx). The synthetic layer is further pinned by anti-ferromagnetic layer (AFM). The pinning field, or exchange anisotropy, is related to the exchange coupling between an antiferromagnetic (AFM) layer and a ferromagnetic sub-layer (AP2) [2].
In a conventional angle sensor, the sensing elements are four long AMR (anisotropic magneto-resistance) [1] stripes oriented in a diamond shape with the ends connected together by metallization to form a Wheatstone bridge, as shown in FIG. 2. The top and bottom connections of the four identical elements are given a direct current stimulus in the form of a supply voltage (Vs), with the remaining side connections to be measured as ΔV. With no magnetic field present, the side contacts should be at the same voltage. To have the elements' magnetization directions align with an externally applied magnetic field, the latter must be large enough to saturate the permalloy material.
With the AMR elements connected in this fashion to form the Wheatstone bridge, the side contacts will produce a different voltage (ΔV) as a function of the supply voltage, MR ratio, and the angle; which is the angle between the element current flow and element magnetization (M). One set of this bridge only provides a measurement of angles ranging from −45 degree to +45 degree. Combined with a second bridge which is oriented 45-degree in rotation from the first set, a wide range of angles, from −90 degree and +90 degrees, can be measured.
In this prior art design, due to the characteristic of the AMR effect, one of the AMR Wheatstone bridges only detects within a 90-degree angle range while two AMR Wheatstone bridges with 45-degrees orientation difference only allow measurement over a 180-degree angle range. In order to measure a full 360-degree angle, an additional Hall sensor must be used in combination with the two Wheatstone bridges.